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Great Debate on GRB Composition: A Case for Poynting Flux Dominated GRB Jets. Bing Zhang Department of Physics and Astronomy University of Nevada, Las Vegas March 6, 2011 In “Prompt Activity of Gamma-Ray Bursts” Raleigh, North Carolina. Reference: Zhang & Yan (2011, ApJ , 726, 90).
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Great Debate on GRB Composition:A Case for Poynting Flux Dominated GRB Jets Bing Zhang Department of Physics and Astronomy University of Nevada, Las Vegas March 6, 2011 In “Prompt Activity of Gamma-Ray Bursts” Raleigh, North Carolina Reference: Zhang & Yan (2011, ApJ, 726, 90)
Fingerprint/ footprint/ Smoking gun: GRB 080916C(Abdo et al. 2009, Science)
Standard Fireball Shock Model central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks
Predicted spectra Meszaros & Rees (00) Zhang & Meszaros (02, unpublished) Daigne & Mochkovitch (02)
Expected photosphere emission from a fireball(Zhang & Pe’er 09) Sigma: ratio between Poynting flux and baryonic flux: = LP/Lb: at least ~ 20, 15 for GRB 080916C Confirmed by Fan (2010) with a wider parameter space study.
The simplest fireball model does not work! Modified fireball models
Modified Fireball Model (1) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks
Magnetic acceleration? High initially, but quickly low ? • MHD model, magnetic pressure gradient accelerate ejecta, Poynting flux can be (partially) converted to kinetic energy (Vlahakis & Konigl 2003; Komissarov et al. 2009; Tchekhovskoy, Narayan & McKinney 2010; Granot, Komissarov & Spitkovsky 2011; Lyubarsky 2010) • The conversion efficiency is low • With external confinement (e.g. stellar envelope), the efficiency can be higher, but the flow can still have a moderately high σ in the emission region. Talks by Narayan, Tchekhovsky, McKinney, Giannios …
Difficulties/issues of the internal shock model • Missing photosphere problem • Low efficiency • Fast cooling problem • Electron number excess problem • Ep – Eiso (Liso) correlation inconsistency
Modified Fireball Model (2) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from internal shocks and photosphere Afterglow: from external shocks
The band function is emission from the photosphere • Dissipated photosphere with upscattering (Thompson 1994; Rees & Meszaros 2005; Ghisellini et al. 2007; Pe’er et al. 2006; Giannios 2008; Beloborodov 2010; Lazzati & Begelman 2010; Pe’er & Ryde 2010; Ioka 2010; Metzger et al. 2011) • Two problems: • Cannot reach > 1 GeV • Low energy spectral index is too hard α ~ -1 ? ? α ~ (+0.4 - +1) Beloborodov (2010); Mizuta et al. (2010); Deng & Zhang (poster)
The band function is emission from the photosphere • Superposition (Toma et al. 2010; Li 2009)? • Contrived fine-tuning • Seems not supported by data (Binbin’s talk) ? ?
The band function is emission from the photosphere • Synchrotron + photosphere (Giannios 2008; Vurm’s talk; Beloborodov’s talk)? • Predict bright optical emission • Prompt optical data (Yost et al.) do not support this possibility (Shen & Zhang 09) Giannios (2008)
Fingerprint/ footprint/ Smoking gun: Preece’s talk
Diverse Ejecta Composition:Thermal emission in GRB 090902B! Ryde et al. (2010); B.-B. Zhang et al. (2011) - Bin-Bin’s talk This is a Paczynski-Goodman “fireball”!
GRB 090902B (probably also 090510) • At least GRB 090902B is a fireball (probably also 090510) • Rare: 2 of 17 LAT GRBs (B.-B. Zhang’s talk) • Photosphere emission looks quasi-thermal. Band function is not superposition of photosphere emission • Photosphere model must address diversity of Comptonization
Constrain Emission Site • It is very difficult to constrain the sub-MeV/MeV emission radius R directly • There are three ways to constrain R • The emission radius of X-ray steep decay phase can be estimated. If Rx= R, then R can be constrained • The emission radius of prompt optical emission Ropt can be constrained by the self-absorption limit. If Ropt= R, then R can be constrained • The emission radius of the GeV photons RGeV can be constrained by the pair-production limit. If RGeV = R, then R can be constrained
Method One: X-Rays • If the steep-decay phase of the X-ray tail is defined by the high-latitude emission, one has: ttail R tail j Kumar et al. 07 Lyutikov, 06 R,X> 1015 cm GRB Rj2/2
Method Two: Optical “Tracking” optical band detection constraints the self-absorption frequency and, hence, the emission radius GRB 050820A Shen & Zhang (08): R,opt > several 1014 cm Vestrand et al. 2006a,b
GRB 080319B: naked-eye GRB(Racusin et al. 2008) Spectrum: two distinct spectral components Lightcurve: optical roughly traces gamma-rays
Syn + SSC model for GRB 080319B (Racusin et al. 2008; Kumar & Panaitescu 2008) E2 N(E) Y ~ 10 Y2 ~100 Klein-Nishina cut-off Y ~ 10 R,opt ~ 1016 cm E Esyn~20 eV ESSC2st ~25 GeV ESSC1st ~650 keV
Method Three: GeV Pair cutoff feature depends on both bulk Lorentz factor (Baring & Harding 1997; Lithwick & Sari 2001) and the unknown emission radius (Gupta & Zhang 2008) 100 200 400 600 800 1000 Gupta & Zhang 2008
Radius constraints(Zhang & Pe’er 09) Emission must come from a large radius far above the photosphere.
A Poyting-Flux-Dominated Flow:Kill Three Birds with One Stone • Invoking a Poynting flux dominated flow can explain the lack of the three expected features • Non-detection of the pair cutoff feature is consistent with a large energy dissipation radius • Non-detection of the SSC feature is naturally expected, since in a Poynting flux dominated flow, the SSC power is expected to be much less that the synchrotron power • Non-detection of the photosphere thermal component is consistent with the picture, since most energy can be retained in the form of Poynting flux energy rather than thermal energy
Counter-arguments: Hide the thermal component Change R0? • R0 = c t ~ 3109 cm (based on the observed minimum variability, and the collapsar scenario) • If R0 is smaller (106 cm - not observed, not favored for a massive star progenitor), the thermal temperature is higher, may be hidden below the non-thermal component. • But it does not work - a Poynting flux dominated flow is still needed to hide the thermal component (Fan 2010)
A Model in the High-σ Regime:The ICMART Model(Internal Collision-induced MAgnetic Reconnection & Turbulence)(Zhang & Yan 2011, ApJ, 726, 90) Basic Assumptions: • The central engine launches a high-σ flow. The σ is still ~ (10-100) at R ~ 1015 cm. • The central engine is intermittent, launching an outflow with variable Lorentz factors (less variable in σ).
ICMART Model Zhang & Yan (2010) (a) Initial collisions only distort magnetic fields (b) Finally a collision triggers fast turbulent reconnection - An ICMART event (a broad pulse in GRB lightcurve)
Distance Scales in the ICMART Model Emission suppressed GRB At most 1/(1+σ) energy released At most 1/(1+σ) energy released 1/(1+σend) energy released photosphere R ~ 1011 - 1012 cm 0 early collisions R ~ 1013 - 1014 cm ~ 1- 100 ICMART region R ~ 1015 - 1016 cm ini ~ 1- 100 end 1 External shock R ~ 1017 cm 1 central engine R ~ 107 cm = 0 >> 1
GRB ejecta is turbulent in nature Reynold’s number: Magnetic Reynold’s number: Magnetic fields can be highly distorted and turbulent if turbulent condition is satisfied
λ L
Turbulent Reconnection is needed to power GRBs In order to reach GRB luminosity, the effective global reconnection rate has to be close to c . Relativistic Sweet-Parker reconnection speed is << c (Lyubarsky 2005). Turbulent reconnection (Lazarian & Vishniac 1999) can increase reconnection speed by a factor L/λ .
Multiple collisions can distort field lines and eventually trigger turbulence in a high-σ flow Required condition from the observations (reach GRB luminosity): Condition for relativistic turbulence (I): relativistic shock Condition for relativistic turbulence (II): relativistic reconnection outflow
Features of the ICMART model Zhang & Yan (2011) • Carries the merits of the internal shock model (variability related to central engine) • Overcomes the difficulties of the internal shock model (carries the merits of the EM model) • High efficiency ~ 50% • Electron number problem naturally solved (electron number is intrinsically small) • Turbulent heating may overcome fast cooling problem • Amati relation more naturally interpreted (larger R, smaller , easier to have reconnection “avalanche”) • No missing photosphere problem
Two-Component Variability in the ICMART Model Consistent with data: Shen & Song (03) Vetere et al. (06) Poster: H. Gao, B.-B. Zhang & B. Zhang slow variability component related to central engine fast variability component related to turbulence
General picture • 15/17 LAT GRBs are Band only • 2/17 with extra PL component • Applicability of ICMART: at least GRB 080916C, probably most Band-only GRBs